It was reported in 1926 by H. Fricke and S. Morse in the article "The electric capacity of tumours of the breast", (1926) J. Cancer Res. 16, pp. 310-376 that the electrical properties of breast tumors differ significantly from healthy tissue. Until now it has not been possible to use these properties to detect breast tumors in a manner useful in a clinical setting. However, some laboratory and imaging techniques have evolved.
Electrical Impedance Tomography (EIT) is an imaging methodology that is based upon electrical conductivity or impedance contrasts within the human body. EIT has been the subject of considerable attention recently but, generally, methods used for image recovery have yielded only low-resolution results.
U.S. Pat. No. 4,539,640, issued Sep. 3, 1995, to inventors Bradley Fry and Alvin Wexler (referred to below as the Wexler et al patent), and the article by A. Wexler, B. Fry and M.R. Neuman, entitled "Impedance-Computed Tomography: Algorithm And System", Applied Optics, Vol. 24, No. 23, pp. 3985-3992, describe a method and embodiment of a system that solved electromagnetic field equations that govern current flow in a conductive medium, and concurrently extracted an image of the interior of the medium based on the electric current conductivity (and, more generally, specific impedance) distribution in the medium. This provided a methodology for the correct mathematical solution of the inverse (imaging) problem and construction of electronic equipment for this purpose. The method also provided for the accommodation of a great number of pixels through the use of sparse-matrix techniques. U.S. Pat. No. 4,539,640 is incorporated herein by reference.
This methodology embodies a number of technological features, e.g. uses a well-conditioned, least-squares method, uses true three-dimensional field solving for images and conductivity values and for identification of characteristic pathologies, is applicable to complex impedance as well as to purely conductive imaging, it allows for application of known CT-based reconstruction methodologies and image processing operations between iterations, only simple contact of electrodes to the skin or to geophysical or other surfaces is sufficient to provide for contact and spreading resistance, it results in a sparse-matrix formulation for high-definition imaging, parallel data acquisition may be performed through frequency multiplexing for speed, and parallel image reconstruction may conveniently be accommodated.
EIT uncovers objects within a host medium by solving for resistivity (or, more generally, specific impedance) distributions within the body. Various techniques have previously been used that treat the flow of applied electrical currents as though they behave in a manner similar to X-ray beams. With this assumption, algebraic reconstruction techniques (ART) originally described by Gordon, Bender and Herman in the article "Algebraic Reconstruction Techniques (ART) For Three Dimensional Electron Microscopy and X-Ray Photography", (1970), J. Theor. Biol. 29, pp 471-481, have been employed by others to uncover a crude approximation of the EIT image.
ART finds wide and accurate use in applications of computed tomography other than EIT. Because electrical currents between any two electrodes flow throughout the body and do not follow ray-like paths, a straightforward application of ART is inappropriate. Therefore, as confirmed by R. H. T. Bates, G. C. McKinnon and A. Seager in the article "A Limitation On Systems For Imaging Electrical Conductivity Distributions", (1980), IEEE Biomed Eng. BME-27, pp. 418, a comprehensive field-solving approach is needed as part of the EIT imaging process.
A considerable body of EIT work has been done at the University of Sheffield in the U. K. Smith, Freeston and Brown describe their Applied Potential Tomography (APT) system which uses a weighted back-projection technique, in "A Real-Time Electrical Impedance Tomography System For Clinical Use--Design And Preliminary Results", (1995), IEEE Trans. Biomed. Eng. BME-42, pp. 133-140.
Guardo et al describe a back-projection reconstruction method which could detect a 3 ml plastic sphere at the centre of a torso-sized cylinder of saline, in "An Experimental Study In Electrical Impedance Tomography Using Backprojection Reconstruction", (1991), IEEE Trans. Biomed. Eng. 38 (7), pp. 617-627. This translates to approximately a 1.5 cm sphere in a 50 cm cylinder.
Shahidi, Guardo and Savard in "Electrical Impedance Tomography: Computational Analysis Based On Finite Element Models Of The Human Thorax With Cylindrical And Realistic Geometries", (1995), Annals Biomed. Eng. 23 (1), pp. 61-69, report that three-dimensional finite element method simulation results show that "a 10 ml edema region with a conductivity equal to that of blood can be detected at a 40 dB signal-to-noise ratio (SNR)", and further: "Detection of a smaller volume, in the order of 2 ml, should be possible by improving either the instrumentation to achieve 60 dB SNR or the performance of the reconstruction methods". These results, scaled to the size of the breast, indicate that even small breast tumors (less than 4 mm in diameter) should detectably alter surface potentials.
It should be noted that detection is not image reconstruction but it is a necessary precondition. Guardo's research team has demonstrated that a practical, measurable signal is available for use in EIT.
Henderson and Webster presented a means for displaying isoadmittance contours of the chest, Tasto and Schomberg described an impedance imaging technique that considers curved current flux tubes and uses back-projection techniques, Lytle and Dines report on the use of impedance techniques for geophysics applications and Price further discusses medical applications and techniques (Henderson, R. and Webster, J. (1978) "An Impedance Camera For Spatially Specific Measurements Of The Thorax", IEEE Trans. Biomech. Eng. BME-25, pp. 250; Tasto, M. and Schomberg, H. (1981) "Method Of And Device for Determining Internal Body Structure", Washington, D.C., U.S. Pat. No. 4,263,920; Lytle, R. J. and Dines, K. A. (1978) "An Impedance Camera: A System for Determining the Spatial Variation of Electrical Conductivity", (1978) Livermore, Calif.: Lawrence Livermore Laboratory Report UCRL-52413; and Price, L. R. (1979) "Electrical Impedance Computed Tomography (ICT): New Imaging Technique", IEEE Trans. Nucl. Sci. NS-26, 2736.
An alternative method as described in the aforenoted U.S. Pat. No. 4,539,640, involves the application of currents to the body and successive measurement of surface potentials. This image recovery method involves the solution of the Poisson/Laplace equation while employing sparse-matrix techniques.
Dijkstra, A. M., B. H. Brown, A. D. Leathard, N. D. Harris, D. C. Barber, and D. L. Edbrooke "Review: Clinical Applications Of Electrical Impedance Tomography", (1993), J. Med. Eng. Technol. 17 (3), pp. 89-98 discuss clinical applications of EIT. They review the conductivity of tissues at around 50 kHz, and show the large contrasts that exist. In their view, " . . . the major disadvantage is the poor spatial resolution which is only about 10% of the diameter of the body. It seems likely that this may be improved to 5% (1 cm in a body of 20 cm diameter) and at this point it begins to be similar to that offered by a gamma camera. We should therefore regard the technique as a monitor of body function and not as an anatomical imaging method."
We believe that this conclusion is pessimistic, as the present invention provides a practical EIT method and apparatus with high resolution that can be used in a clinical setting.